The present invention relates to electrochemical cells and more particularly relates to pressure relief structures for electrochemical cell containers.
Electrochemical cells such as galvanic cells for producing electric power typically are arranged in sealed containers such as can-like structures including tubular side walls and generally planar end walls. The container protects the interior components of the cell from the environment, protects surrounding structures from corrosive materials within the ell, and typically also serves as an electrical conductor in the cell structure. Certain types of electrochemical cells, however, can evolve substantial quantities of gases under abnormal conditions such as short circuited operation or exposure to extremely high ambient temperatures, as during a fire. Accordingly, the sealed containers for such cells have been provided heretofore with pressure relief devices. Typically, such pressure relief devices have included one or more frangible portions of the container wall adapted to break in a controlled fashion under the influence of excessive internal pressures within the container.
The pressure relief devices have ordinarily been disposed in the end walls of the container. For example, Markin et al, U.S. Pat. No. 4,476,200 discloses a galvanic cell container having a polymeric end wall with a frangible, polymeric disc adjacent an edge of the end wall. Tsuchida et al, U.S. Pat. No. 4,237,203 also employs a rupturable polymeric gasket. Tamura et al, U.S. Pat. No. 4,256,812 shows a galvanic cell container with an end wall having score lines extending in a cruciform pattern, so that the end wall will rupture along the cruciform score lines when an excessive pressure is attained within the cell. Dey et al, U.S. Pat. No. 4,533,609 utilizes an end wall defined by a cap formed separately from the tubular side wall of the container. The cap-defining end wall is fitted to the side wall so that excessive pressures within the container will blow the cap entirely away from the side wall. Also, Lees, U.S. Pat. No. 4,484,691 discloses a container having deformed, wrinkled portions arranged so that internal pressures within the container tend to flatten the wrinkled portions. Small score lines are disposed at the ends of the wrinkled portions so that when the container is subjected to an excessive internal pressure, the container wall ruptures at these score lines, the rupturing action being assisted by the deformed wrinkled portions. As taught by Lees et al, the deformed, wrinkled portions may be arcuate and may be disposed on an end wall of the container, or may be disposed on the tubular side wall of the container.
Although venting or pressure relief systems utilized heretofore can indeed relieve excessive pressures caused by gas evolution within an electrochemical cell, the systems utilized heretofore have created additional problems. With certain of the venting systems utilized heretofore, solid components of the cell can escape through the opening provided for pressure relief. These components may be projected with substantial energy from the cell and may pose hazards to personnel and equipment in the vicinity of the cells. Moreover, solid components of the interior cell structure projected from the pressure relief opening by the flowing gas can carry corrosive or otherwise dangerous materials out of the cell and hence create a hazard to surrounding structures and personnel.
Moreover, the fluids issuing from the pressure relief opening create a substantial reaction force on the cell container itself, in much the same way as the exhaust gases issuing from a rocket motor apply a reaction force to the rocket. The reaction forces applied by the gases tend to accelerate the cell. With a violent release of gases as may be encountered with a severe overpressure condition, the cell can become a dangerous flying projectile.
There have accordingly been significant needs heretofore for improvements in pressure relief devices for electrochemical cells. Moreover, there have been additional needs for electrochemical cell pressure relief devices which can be incorporated in the cell at minimal cost and which will nonetheless function reliably.